1. Field of the Invention
This invention relates to the field of start-up circuits, and particularly to start-up circuits which, in response to a start-up signal, provide a means of generating fixed bias currents suitable for use in other circuits.
2. Description of the Related Art
Analog circuitry typically employs a number of fixed current sources, used, for example, to provide bias currents and establish voltage or current limits. An example of a circuit that requires a fixed bias current is the voltage regulator shown in FIG. 1. An input voltage V.sub.in is connected to the emitter 10 of a pass transistor 12, typically a pnp bipolar transistor, and an output voltage V.sub.out is taken at the transistor's collector 14 and drives a load R.sub.load. The output voltage is regulated by controlling pass transistor 12 via its control input 16. Regulation is accomplished with a feedback loop: the output voltage is fed back to a loop amplifier 18, usually via a voltage divider 20. A voltage reference V.sub.ref is also connected to the amplifier, which produces an output proportional to the difference between its two inputs. The amplifier's output is connected to a drive circuit 22, which produces the drive signal that controls the pass transistor 12.
The drive circuit 22 includes a drive transistor Q.sub.drive, which produces a drive current i.sub.drive used to control pass transistor 12. The base of Q.sub.drive is driven from the emitter of a buffer transistor Q.sub.b, which is in turn driven by a bias current i.sub.bias. Q.sub.b 's base is also connected to the collector of an inverting transistor Q.sub.inv, which is driven by the output of amplifier 18 via an emitter follower transistor Q.sub.a. A resistor Ra is connected between the base of Q.sub.inv and the base of Q.sub.drive. The emitters of transistors Q.sub.b and Q.sub.a are pulled down with respective current sources i2 and i1.
A well-controlled, known bias current i.sub.bias is important to the operation of the regulator. Drive current i.sub.drive is controlled by amplifier 18, until it reaches a maximum value that depends in part on i.sub.bias. Assuming that i1 and Ra are fixed values, the maximum drive current i.sub.drive (max.) is given by: EQU i.sub.drive (max.)=i.sub.bias .times.e.sup.(i1.times.Ra)/(kT/q)
When i.sub.drive (max.) is appropriately set, it protects the pass transistor from being overdriven. In this exemplary analog circuit, establishing a precise drive current limit requires that i.sub.bias be a known, fixed value. A well-controlled i.sub.bias is also important when i.sub.drive is below the maximum, so that amplifier 18 can provide as much drive as may be needed for normal operation.
Many analog circuits, including some voltage regulators, are designed to become active upon receipt of a "start-up" signal, which can be a voltage, a current, or a logic signal, for example. Start-up signals are often derived from unregulated voltage sources, making the generation of fixed bias currents directly from the start-up signal impractical. A need exists for a circuit that can generate multiple known, fixed bias currents upon receipt of an unregulated or varying start-up signal.